1. Field of the Invention
The present invention relates to a light-emitting unit, an illumination device for illuminating a document, and a contact-type image reading device (i.e., an image sensor) incorporating the illuminating device therein.
2. Description of the Prior Art
A contact-type image sensor is used as a device for reading a document using facsimile terminal equipment, a copying machine, an image scanner, etc. This contact-type image sensor is provided with a line illumination device for linearly illuminating a document surface over a main scanning range.
The line illumination device using a light guide is known and disclosed, for example, in Japanese Unexamined Patent Publication No. HEI 8-163320 (1996) and Japanese Unexamined Patent Publication No. HEI 10-126581 (1998) (Japanese Patent No. 2999431) in which the line illumination device using a bar-shaped or plate-shaped light guide and an image reading device using such a line illumination device are described.
FIG. 15 is a perspective view of a conventional bar-shaped line illumination device in which a bar-shaped light guide 101 is housed within a white casing 100 and a light-emitting unit 102 is secured to one end of the bar-shaped light guide 101. Light emitted from the light-emitting unit 102 is emitted toward a document surface from a light-emitting surface of the bar-shaped light guide 101 exposed from the casing 100 while reflecting the light within the bar-shaped light guide 101.
Structure of the light-emitting unit 102 is shown in FIG. 16, in which a light-emitting unit board 110 made of resin is provided with a window section 103. A light-emitting device 105 is bonded with a lead frame 104 facing the window section 103 by an adhesive agent 106 and connected to the lead frame 104 by a metal wire 107, wherein the window section 103 is sealed by transparent resin.
The lead frame 104 comprises a lead terminal section serving as an outer connecting terminal, an inner lead section, and a light-emitting device mounting and connecting section exposed within the window section 103. In the prior art, phosphor bronze is used as material for the lead frame 104, and silver paste formed by mixing resin with silver powder is used as material for the adhesive agent 106. An outer connecting terminal 104a of the lead frame 104 guided out of the light-emitting unit board 110 is adapted to solder the corresponding electric circuit.
If the brightness of the line illumination device is increased to increase the brightness of the illumination light for illuminating the reading surface of a document, it is possible to easily speed up the image reading. However, when an electric current sent to the light-emitting device is increased to increase the brightness of the line illumination device, calorific value of the light-emitting device is also increased accordingly. In this case, luminous efficiency decreases and as a result, there is a problem that it is difficult to increase the brightness.
When the electric current is supplied to the light-emitting device, junction temperature rises at the same time when light is emitted (i.e., heat is generated from the light-emitting device itself). Generated heat is transmitted or escapes to the side of the light-emitting unit board and finally radiated in the air. Accordingly, the rise of the junction temperature depends on the heat radiation characteristics of the light-emitting unit board and is substantially proportional to the supplied electric current. Namely, if the heat radiation characteristics of the light-emitting unit board are good or satisfactory, the rate of rise in the junction temperature becomes small.
On the other hand, to send an electric current to the light-emitting device (i.e., to turn on a light) at a high temperature leads to acceleration of deterioration of the light-emitting device. To lengthen the life, it is desirable that the temperature rise of the light-emitting device be controlled as much as possible. Accordingly, the better the heat radiation performance of the light-emitting unit board, the larger the maximum current which can be supplied to the light-emitting unit. The heat radiation performance of the light-emitting unit board varies with the shape, material or the like. In the prior art, phosphor bronze has been used for the material of the lead frame of the light-emitting unit board.
Namely, if the same level of brightness as before is required, the phosphor bronze can be used as the material for the lead frame, but the heat conductivity is not enough to obtain the higher or further brightness.
Solder used in the case where the outer connecting terminal of the lead frame is soldered to the corresponding electric circuit is generally an alloy of tin (Sn) and lead (Pb). However, there is a tendency to use the solder which does not contain the lead (i.e., lead-free solder) from the environmental point of view these days.
On the other hand, since material resin for the light-emitting unit board must stand the heat when soldered, polybutylene terephthalate (PBT) has been used in the prior art. A melting point of the polybutylene terephthalate is 200˜210° C. and there was no problem because the melting point of a conventional solder (i.e., an alloy of tin and lead) is 186° C. However, since the melting point of the lead-free solder is 210˜230° C., heat resistance is not enough in the case of polybutylene terephthalate (PBT).
By increasing the brightness of the light-emitting unit which is incorporated in the line illumination device, it is possible to speed up image reading, but there is caused a new problem that the luminous efficiency decreases when the temperature is high.